Index: bin/pkill/tests/Makefile
===================================================================
--- bin/pkill/tests/Makefile
+++ bin/pkill/tests/Makefile
@@ -2,6 +2,11 @@
 
 .include <bsd.own.mk>
 
+# The test programs in this directory conflict with each other and also cause
+# spurious failures in other tests if run concurrently.  Run them exclusively
+# for the time being.
+TEST_METADATA+=			is_exclusive=true
+
 TAP_TESTS_SH=			pgrep-F_test
 TAP_TESTS_SH+=			pgrep-LF_test
 TAP_TESTS_SH+=			pgrep-P_test
Index: lib/libc/tests/sys/Makefile
===================================================================
--- lib/libc/tests/sys/Makefile
+++ lib/libc/tests/sys/Makefile
@@ -33,9 +33,13 @@
 NETBSD_ATF_TESTS_C+=		mmap_test
 NETBSD_ATF_TESTS_C+=		mprotect_test
 NETBSD_ATF_TESTS_C+=		msgctl_test
+TEST_METADATA.msgctl_test+=	is_exclusive=true
 NETBSD_ATF_TESTS_C+=		msgget_test
+TEST_METADATA.msgget_test+=	is_exclusive=true
 NETBSD_ATF_TESTS_C+=		msgrcv_test
+TEST_METADATA.msgrcv_test+=	is_exclusive=true
 NETBSD_ATF_TESTS_C+=		msgsnd_test
+TEST_METADATA.msgsnd_test+=	is_exclusive=true
 NETBSD_ATF_TESTS_C+=		msync_test
 NETBSD_ATF_TESTS_C+=		nanosleep_test
 NETBSD_ATF_TESTS_C+=		pipe_test
Index: tests/etc/rc.d/Makefile
===================================================================
--- tests/etc/rc.d/Makefile
+++ tests/etc/rc.d/Makefile
@@ -3,5 +3,6 @@
 TESTSDIR=	${TESTSBASE}/etc/rc.d
 
 ATF_TESTS_SH+=	routing_test
+TEST_METADATA.routing_test+=	is_exclusive=true
 
 .include <bsd.test.mk>
Index: tests/sys/geom/class/Makefile.inc
===================================================================
--- /dev/null
+++ tests/sys/geom/class/Makefile.inc
@@ -0,0 +1,7 @@
+# $FreeBSD$
+
+# The geom tests conflict with each other and may fail when run concurrently.
+# This is because these tests use the system-wide md(4) resource via
+# mdconfig(1) and the system may run out of virtual devices when running
+# enough of these tests at once.  Run them exclusively for now.
+TEST_METADATA+=	is_exclusive=true
Index: tests/sys/netinet/Makefile
===================================================================
--- tests/sys/netinet/Makefile
+++ tests/sys/netinet/Makefile
@@ -4,6 +4,7 @@
 BINDIR=		${TESTSDIR}
 
 ATF_TESTS_SH=	fibs_test
+TEST_METADATA.fibs_test+=	is_exclusive=true
 
 PROGS=	udp_dontroute tcp_user_cookie